Radionuclide emission scintillation cameras, also called Anger cameras, are used to image the distribution of gamma-ray radioactive material within a body part or organ, such as the brain or the breast, for example, for diagnostic purposes. A source of penetrating radiation is administered to the patient, which typically consists of a pharmaceutical tagged with a gamma-ray emitting radionuclide (radiopharmaceutical) designed to go to and deposit in the organ or elements of the body under diagnostic examination, such as, for example, in the detection of a tumor. Gamma-rays emitted by the radiopharmaceutical are received and detected by the camera, the position of each detected ray event is determined, and the image of the radioactivity distribution in the organ or other body part is constructed by known techniques from an accumulation of events.
Scintillation cameras generally employ an optically continuous crystal of thallium activated sodium iodide, NaI(Tl), as a gamma-ray energy transducer. The energy of the gamma-rays are absorbed in the crystal and are converted to light emissions called scintillation events, each event having an energy proportional to the energy of the absorbed gamma-rays. In conventional cameras, light is transmitted from the crystal to an optically clear glass window through a silicone gel interface that fills a thin separation between the glass window and the crystal. The optical window is part of a container which seals the crystal from air and humidity which would otherwise oxidize the crystal and degrade its optical clarity. An array of photomultiplier (PM) tubes is optically coupled to the glass window, typically by means of optically coupling grease, in order to transmit light to photocathodes located on the inner surface of the glass entrance face of each photomultiplier tube. Thus, the scintillation light events must pass sequentially from the NaI(Tl) crystal through the silicone gel interface, glass window, silicone grease interface, and photomultiplier glass before striking the photocathodes within the photomultiplier tubes. The photocathodes serve to convert the light to electrons by the photoelectric effect and the electrons are amplified (multiplied) in the photomultiplier tubes. Amplified signals generated in photomultiplier tubes in the vicinity of the scintillation event are then mathematically combined by known analog or digital means to determine the position and the energy of the gamma-ray absorption in the crystal.
The accurate determination of the energy level and position of the scintillation event requires that the efficiency of transmission of the scintillation light to the photomultiplier tubes be high. Also, since the distribution of the light transferred to an array of photomultiplier tubes from the origin of a scintillation event is used computationally to determine position, light dispersion or deflection which adversely modifies the distribution degrades the position determination. For example, if light is reflected back from an interface and possibly undergoes multiple reflections before striking a photocathode, the position information contained in the photomultiplier signals received is likely to be compromised. Thus, it is important to minimize the probability of back reflections occurring at an interface of optically coupled materials having different indices of refraction by reducing the number of interfaces, matching the indices of refraction as closely as possible, and directing the light by means that will enhance transmission through interfaces to the receiving photocathodes.
The design of a conventional scintillation camera is subject to several optical constraints dictated by the rigid geometry of planar or curved sandwiches of crystal, glass, and intermediate optical coupling materials. By far, the most difficult light transfer occurs at the surface of the crystal leading to the glass interface. The crystal has an index of refraction of about 1.85 and the glass index is typically about 1.54. Currently silicone gel material having an index of about 1.42 is used to couple the crystal to the glass. The gel has good mechanical interfacing characteristics and transmissivity but its index of refraction is a poor match to the crystal and the glass with regard to light transmission through the interfaces. Other materials with indices closer to that of the glass have been employed but their mechanical coupling characteristics are inferior. Light from the crystal which strikes the gel interface at angles of incidence greater than 50 degrees is internally reflected back into the crystal. This internal reflection may be repeated many times between the exit and the entrance faces of the crystal, as in a light pipe, unless the surfaces of the crystal and the internal reflections thereon are diffuse enough to alter the direction of the light rays so as to lower some of the angles of incidence on successive reflections. In the process, the quantity of light transmitted is diminished by light absorption and its distribution, diffused by reflections, results ultimately in degraded energy and position resolution.
Another problem of curved surface cameras, such as cameras of annular, arcuate or hemispherical design, is that the unfavorable expansion coefficients of the crystal, silicone gel coupling material and the glass cause each of them to expand and contract in opposition to the others with increasing or decreasing temperatures. As the temperature increases, pressure is put on the glass and crystal possibly causing fracture. As the temperature drops, the silicone gel may de-couple from either the glass or the crystal and light may be prevented from passing from the crystal to the glass. Consequently, curved surface cameras made by conventional methods may have a limited operating temperature range between approximately 60.degree. F. to 80.degree. F., for example. Shipping temperature range is also limited which significantly adds to the cost of transportation.
Yet another problem is that crystals for scintillation cameras must generally be constructed from a single optically continuous crystal material. Otherwise an optical discontinuity will result in back reflections at the discontinuity interfaces which disrupt the direction of light transmission to the photomultipliers generally making it impossible to image by usual scintillation camera methods. In some instances, such as the construction of a curved surface camera, for example, it is particularly costly to construct an annular crystal system using an optically continuous single crystal annulus.